Preheat oven to 350 and line 2 baking sheets with parchment paper. Set aside.
Add cake mix, flour, water, oil and eggs to a mixing bowl and mix well until evenly combined. Divide batter in half and color one red and one blue.
Use a 2 tablespoon portion scoop to scoop batter onto prepared baking sheets, keeping about 3" apart. Use the back of the scoop to flatten the pies to about ½" thick and about 2-½" around.
Bake for 12 minutes.
Remove from oven and let stand on the baking sheet to cool for a few minutes before transferring to a cooling rack to cool completely.
Once cooled, place a large dollop of frosting onto the flat side of the blue round pie.
Add a red pie, on top, keep the flat sides together.
Sprinkle red, white and blue sprinkles on the frosting. Repeat until all whoopie pies are assembled and decorated.
Serve immediately or store any leftover whoopie pies covered and in the refrigerator.